Casalini R, Ransom T C
Naval Research Laboratory, Chemistry Division, Washington, DC 20375-5342, USA.
Soft Matter. 2020 May 21;16(19):4625-4631. doi: 10.1039/d0sm00254b. Epub 2020 May 5.
Since its initial discovery more than fifteen years ago, the thermodynamical scaling of the dynamics of supercooled liquids has been used to provide many new important insights in the physics of liquids, particularly on the link between dynamics and intermolecular potential. A question that has long been discussed is whether the scaling exponent γ is a constant or does it depends on pressure. An alternative definition of the scaling parameter, γ = ∂ ln T/∂ ln ρ| has been presented in the literature, and has been erroneously considered equivalent to γ. Here we offer a simple method to determine the pressure dependence of γ using only the pressure dependence of the glass transition and the equation of state. Using this new method we find that for the six nonassociated liquids investigated, γ always decreases with increasing pressure. Importantly in all cases the value of γ remains always larger than 4. Liquids having γ closer to 4 at low pressure show a smaller change in γ with pressure. We argue that this result has very important consequences for the experimental determination of the functional form of the repulsive part of the potential in liquids. Comparing the pressure and temperature dependence of γ and γ we find, contrary to what has been assumed in the literature to date, that these two parameters are not equivalent and have very different pressure and temperature dependences.
自15多年前首次发现以来，过冷液体动力学的热力学标度已被用于在液体物理学中提供许多新的重要见解，特别是在动力学与分子间势之间的联系方面。长期以来一直讨论的一个问题是，标度指数γ是一个常数，还是它取决于压力。文献中提出了标度参数的另一种定义，γ = ∂ ln T/∂ ln ρ|，并且被错误地认为等同于γ。在这里，我们提供一种仅使用玻璃化转变的压力依赖性和状态方程来确定γ的压力依赖性的简单方法。使用这种新方法，我们发现对于所研究的六种非缔合液体，γ总是随着压力的增加而减小。重要的是，在所有情况下，γ的值始终大于4。在低压下γ更接近4的液体，其γ随压力的变化较小。我们认为这一结果对于实验确定液体中势的排斥部分的函数形式具有非常重要的意义。比较γ和γ的压力和温度依赖性，我们发现，与迄今为止文献中所假设的相反，这两个参数并不等效，并且具有非常不同的压力和温度依赖性。
